Volume phase reflection holograms in general only reflect a single wavelength of light and thus produce a monochromatic reconstructed image. To produce multicolor reconstructed images, one must image a series of holograms in the same recording material. This is a difficult and time consuming process.
In one approach, a panchromatic recording material is selected that is sensitive to a broad spectrum of visible radiation. A series of holograms is imaged in the material by exposure to different wavelengths of light. Weber et al., for example, disclose imaging a photopolymer holographic recording material with 647 nm, 528 nm, and 476 nm light to produce a three color holographic mirror in Practical Holography IV, Proc. SPIE, 1212, pp 30-39 (1990). This approach, however, requires incorporation of sensitizing dyes to achieve response to the various wavelengths. Each dye must fade with time, or the dye itself will impart color to the hologram. Also, the intensity of each exposure must be carefully controlled to consume only a portion of the monomer available in the recording material so that monomer will be available for subsequent exposures to create the other colors. Thus, the resulting colored hologram may not be as bright as desired.
In another approach, all exposures are conducted at the same wavelength, with the recording material being progressively swollen or shrunk following each exposure. T. J. Cvetovich, for example, describes a process wherein a recording medium is exposed sequentially to a series of master holograms in Applications of Holography, Proc. SPIE, 523, pp 47-51, 1985. The recording medium is swollen with triethanol amine prior to the first exposure and progressively dried prior to each succeeding exposure. It is difficult to reproducibly process holograms using this approach, however, because of the difficulty of reliably controlling the amount of swelling or shrinkage in wet processing.
In Smothers, U.S. Pat. No. 4,959,283, a process is disclosed for altering the wavelength of response of a hologram. Following exposure and development of the hologram, a dry film diffusion element is laminated to it. Material is allowed to diffuse into or out of the hologram altering its wavelength of response. However, this process only changes the wavelength of response of the entire hologram and does not produce a multicolor hologram.
Thus, there is a continuing need for a practical method for forming multicolor holograms.